This invention relates to steam turbines and to steam and gas turbine combined cycle power generators. In particular, this invention relates to methods to rapidly increase the power output of a steam turbine.
Power generating equipment connected to a power distribution grid is occasionally required to quickly increase power output. Rapid power increases may be needed to balance power in a utility electrical power grid or to manage grid frequency variations. However, it is often difficult for utility steam power generators to immediately increase their power output levels to satisfy demands for quick power.
Power generators, such as steam turbines and combined cycle steam and gas turbine units, are large turbomachines with generation and operation limits. Gas turbines may provide fast additional power. Steam turbines traditionally provide a slower response to power change requests.
To change the power output of a steam turbine, a substantial time period, e.g., 2 to 5 minutes, is generally needed to: increase the fuel to boiler to provide extra heat into the boiler, increase the flow or fluid temperature in the boiler tubes, produce additional steam in the boiler, apply the added steam to the steam turbine, and generate extra power. These steps to increase the power of a steam turbine are used to increase the normal steady-state power output of the turbine. However, there can be a need for a technique to quickly and temporarily increase the power output of a steam turbine.
When a rapid increase in steam turbine power is required, a conventional method involves opening control valves to admit more steam flow to the turbine. This method works if, when the demand for more power is received, the steam control valves are partially closed and pressure in the superheater is substantially higher than that needed to drive the steam turbine at the power level existing immediately prior to the demand. This conventional method is not applicable if the steam turbine is already operating in a steady state condition with the control valves wide open or there is no excess steam pressure in the superheater when the demand for quick power is received. Further, this conventional method increases the flow of steam without increasing the enthalpy amount of steam being generated.
Another conventional method to rapidly (albeit briefly) increase the output of a steam turbine is to spray water in attemperators in the steam superheaters (main or reheat) to generate extra high-pressure steam. Spraying water in the attemperators increases the steam mass flow into the turbine faster than the resulting enthalpy reduction and can relatively quickly increase the power output of the turbine for a limited time. Similarly, and under certain conditions, attemperation by spraying water directly to the hot tubes of the boiler quickly produces additional steam to be fed to the steam generator.
The amount of extra power provided by the attemperation is limited. Moreover, spraying water on the hot tubes of superheater surfaces creates thermal shock that tends to reduce the useful life of these surfaces. Further, attemperation reduces the enthalpy of the steam because steam temperature is reduced when the inner boiler tube surface is cooled by excessive water flow.
The invention may be embodied as a method to rapidly change a power output level of a steam turbine comprising: operating the steam turbine at a continuous power output level including regulating a steam temperature to a constant steam temperature set point; receiving a demand for a rapid change in the power output level of the steam turbine; adjusting the steam temperature set point to a temporary temperature excursion limit selected to achieve the demand for the rapid power change; regulating the steam temperature in the turbine to the temporary temperature excursion limit; operating the turbine at a temporary power output level achieved using the temporary excursion limit; generating power by the turbine using steam regulated to the temporary temperature limit, and restoring the temperature used to regulate the turbine after a predetermined period.
The invention may also be embodied as a method to rapidly change a power output level of a steam turbine comprising: operating the steam turbine at a continuous power output level including regulating a steam temperature to a constant steam temperature set point; receiving a demand for a rapid change in the power output level of the steam turbine; adjusting the steam temperature set point to a temporary temperature excursion limit selected to achieve the demand for the rapid power change; regulating the steam temperature in the turbine to the temporary temperature excursion limit; operating the turbine at a temporary power output level achieved using the temporary excursion limit; generating power by the turbine using steam regulated to the temporary temperature limit; changing the temperature used to regulate the turbine after a predetermined period; promptly after receiving the demand for a rapid power change, increasing a fuel flow to a source of heat production applied to a heat recovery and steam generator (HRSG) which provides steam to the steam turbine; increasing an amount of steam provided by the HRSG to the turbine after the fuel flow increase, and continuing an increase in power output by the steam turbine previously achieved by regulating to the temperature excursion limit by applying the increase in the amount of steam provided to the turbine.
The invention may be further embodied as a steam turbine system comprising: a steam turbine having a steam inlet and steam outlet; a steam circuit having a discharge port coupled to the steam inlet of the steam turbine and an inlet port coupled to the steam outlet, and said steam circuit further comprising a steam generation device for generating steam to flow to the steam inlet of the turbine; at least one adjustable valve in said steam circuit, wherein said valve regulates a condition of steam in the circuit in response to a valve actuation control command; a controller receiving control commands and sensor input regarding steam conditions in the circuit, wherein said controller further comprising an electronically stored control program which, in response to a demand for a rapid power change, (i) generates valve actuation control command to adjust the valve in order to elevate a steam temperature in the circuit to a temporary temperature excursion limit, and (ii) maintains the steam temperature at the temporary temperature excursion limit for no longer than a predetermined period.